Radial velocities and metallicities from infrared Ca ii triplet spectroscopy of open clusters

Abstract

Context. Open clusters are key to studying the formation and evolution of the Galactic disc. However, there is a deficiency of radial velocity and chemical abundance determinations for open clusters in the literature. Aims. We intend to increase the number of determinations of radial velocities and metallicities from spectroscopy for open clusters. Methods. We acquired medium-resolution spectra (R ∼ 8000) in the infrared region Ca ii triplet lines (∼8500 Å) for several stars in five open clusters with the long-slit IDS spectrograph on the 2.5 m Isaac Newton Telescope (Roque de los Muchachos Observatory, Spain). Radial velocities were obtained by cross-correlation fitting techniques. The relationships available in the literature between the strength of infrared Ca ii lines and metallicity were also used to derive the metallicity for each cluster. Results. We obtain V r = 48.6 ± 3.4, −58.4 ± 6.8, 26.0 ± 4.3, and −65.3 ± 3.2 km s −1 for Berkeley 23, NGC 559, NGC 6603, and NGC 7245, respectively. We found [Fe/H] = −0.25 ± 0.14 and −0.15 ± 0.18 for NGC 559 and NGC 7245, respectively. Berkeley 23 has low metallicity, [Fe/H] = −0.42 ± 0.13, which is similar to other open clusters in the outskirts of the Galactic disc. In contrast, we derived high metallicity ([Fe/H] = +0.43 ± 0.15) for NGC 6603, which places this system among the most metal-rich known open clusters. To our knowledge, this is the first determination of radial velocities and metallicities from spectroscopy for these clusters, except NGC 6603, for which radial velocities had been previously determined. We have also analysed ten stars in the line of sight to King 1. Because of the large dispersion obtained in both radial velocity and metallicity, we cannot be sure that we have sampled true cluster members.

The open cluster King 1 in the second quadrant

“…On average, the radial velocities derived here are 15 km s −1 larger than those derived by Carrera et al (2015). A problem with the wavelength calibration of the King 1 spectra analysed by Carrera et al (2015) may explain this discrepancy.…”

“…However, the relations used to derive the stellar metallicities were obtained only for red giant branch stars. Carrera et al (2015) demonstrated that the maximum difference in ΣCa between red clump and red giant branch stars is ∼0.16Å. This implies that the metallicity of a red clump star derived with the red giant branch relationships may be over-estimated by a maximum of ∼0.07 dex, independent of the luminosity indicator used.…”

“…As commented before, Carrera et al (2015) derived radial velocities for ten stars in the line of sight of King 1. The obtained velocity distribution did not show a clear peak in contrast to the sample for other clusters studied in the same paper.…”

On the metallicity of open clusters

“…Friel et al (2002) presented an update of previous investigations by using spectrophotometric indices to derive metallicities of 39 open clusters. Furthermore, from the studies by Carrera et al (2007), Carrera (2012), Carrera et al (2015), and Warren & Cole (2009), we compiled metallicity estimates of 28 open clusters. These authors made use of reduced equivalent widths (W ) of the Ca  triplet lines, which show a linear relation with metallicity.…”

New Cepheid variables in the young open clusters Berkeley 51 and Berkeley 55

“…Again, it lies on the same isochrone as B-type confirmed spectroscopic members (figs 9 and 10 in N12). W2249 is 74 from the centre of NGC 6603; the seven targets -including W2249for which radial velocities supported cluster membership (Carrera et al 2015) lie within 5 of the centre.…”